1. Field of the Invention
The present invention relates to a method and an apparatus for time equalization, and in particular to a method and an apparatus for time equalization employed in modems or the like used at the time of a superfast data transmission with metallic wires.
2. Description of the Related Art
Generally, modems have been used for transmitting data with telephone lines, leased lines, private metallic wires, or the like, while in recent years, enhanced speed has been in demand for the processing of modem.
As a technical field where such modems are used, e.g. a power-line carrier communication can be mentioned. This power-line carrier communication includes an extremely large amount of random noises (white noises) from household electrical appliances such as an inverter, which blocks practical utilization of a high-speed data communication.
As recent countermeasures for such noises, a DMT (Discrete MultiTone) system and an OFDM (Orthogonal Frequency Division Multiplexing) system have been proposed. These DMT system and OFDM system adopt a multicarrier (multichannel) modulation system, that is a technology which avoids a signal from passing through a carrier band rich in noises. However, the multicarrier causes a group delay, as shown in FIG. 12B, in a transmission line on route, even if signals of channels are simultaneously transmitted from a transmission side as shown in FIG. 12A. As a result, arrival times of the channels on a time axis are different from each other on a reception side, as shown in FIG. 12C. For this reason, an inter-channel interference on a time axis arises on the reception side.
Namely, since low-speed square waves are transmitted in the DMT system or the OFDM system as shown in FIGS. 13A-13C, a normal transmission signal (tone) can be obtained, as shown in FIG. 13B, in a stable part of the square wave. However, in a variable part of the square wave, unnecessary bands of each channel have a waveform attenuating at a function of sinx/x as shown in FIGS. 13A and 13C.
Thus, in a line characteristic where the group delay arises, the signals of the channels interfere with each other on a time axis, and the inter-channel interference is avoided only in a flat part of the line characteristic.
On one hand, if a time corresponding to the interference part (i.e. time corresponding to a variable time of the square wave) is masked as a guard time GT as shown in FIGS. 13A-13C, it becomes possible to avoid the inter-channel interference. However, data transmission is disabled for this guard time GT, which makes a high-speed transmission difficult.
Accordingly, equalization between channels on a time axis is necessary in order to solve such a problem of a line group delay.
On the other hand, even in the same apparatus, the static characteristic greatly differs depending on ON/OFF state of power. In household electrical appliances such as a television using a switching power, two transfer functions A (or C) and B (or D) are alternately switched over every 120 Hz (in case a used frequency is 60 Hz) depending on whether the voltage is equal to or less than or more than a fixed value as shown in FIG. 14A. Namely, the transfer functions are switched over 240 times per second.
Thus, if the transfer function varies, a frequency characteristic (amplitude/phase) is divided into a solid line indicating the transfer functions A and C and a dotted line indicating the transfer functions B and D, as shown in FIG. 14B, both being greatly different from each other. Such a variation of the phase characteristic also includes the variation on a time axis.
Accordingly, since a high-speed followup performance is required for the transmission line in a modem or the like used for the power-line carrier communication, not only the high-speed equalization on a time axis as mentioned above, but also equalization on a frequency axis is required. If the equalization on a time axis is performed, it will contribute to the equalization on the frequency axis.
FIG. 15 shows a prior art for realizing the above-mentioned equalization on a time axis and a frequency axis. This prior art is composed of a time equalizer 1, a guard time remover 2, a DMT distributor 3 by an FFT (Fast Fourier Transform) calculation, a frequency equalizer (FEQ) 4, a determining portion (DEC) 5, and a code inverter 6, all being connected in series.
In this arrangement, the time equalizer 1 performs the equalization on a time axis to the received signal, and then the guard time remover 2 removes the guard time added on the transmission side. Furthermore, the DMT distributor 3 performs the FFT. Then, the frequency equalizer 4 performs the equalizations of the carrier amplitude and the carrier phase, and the determining portion 5 determines the code. Then, the code inverter 6 performs the code conversion such as Natural (N)/Gray (G) code conversion, a parallel (P)/serial (S) conversion, and descramble (DSCR) to obtain the received data RD.
In such a prior art, a special training signal is required in order to perform pull-in on a time axis at the time equalizer 1. This training signal requires a long time, and complicated processing for this training.
Namely, since a high-speed followup performance is required for the line characteristic which varies every 120 Hz, as mentioned above, at a multi point of 1:n, it is not possible to provide a long training time at each point, so that simple processing is required.